Method for determining a critical size of an inclusion in aluminum or aluminum alloy sputtering target

ABSTRACT

The present invention relates to a method for determining a critical size for a diameter of an Al 2 O 3  inclusion ( 38 ) in an Al or Al alloy sputter target ( 42 ) to prevent arcing during sputtering thereof. This method includes providing a sputtering apparatus having an argon plasma. The plasma has a plasma sheath of a known thickness during sputtering under a selected sputtering environment of an Al or Al alloy sputter target having an Al 2 O 3  inclusion-free sputtering surface. When the thickness of the sheath is known for a selected sputtering environment, the critical size of an Al 2 O 3  inclusion ( 38 ) can be determined based upon the thickness of the sheath. More specifically, the diameter of an Al 2 O 3  inclusion ( 38 ) in an Al or Al alloy sputter target ( 42 ) must be less than the thickness of the plasma sheath during sputtering under the selected sputtering environment to inhibit arcing.

CROSS-REFERENCE TO RELATED INVENTION

Priority filing benefit of (1) International PCT applicationPCT/US02/10516 filed Apr. 4, 2002, and published under PCT 21(2) in theEnglish language and (2) U.S. Provisional Application Ser. No.60/281,482 filed Apr. 4, 2001.

FIELD OF THE INVENTION

The present invention relates generally to aluminum or aluminum alloysputter targets having aluminum oxide (Al₂O₃) inclusions and, moreparticularly, to a method for determining a critical size of an Al₂O₃inclusion in an aluminum or aluminum alloy sputter target to inhibitbipolar arcing during sputtering thereof.

BACKGROUND OF THE INVENTION

The presence of an Al₂O₃ inclusion in an Al or Al alloy sputter targetcan result in arcing when the target is sputtered in a sputteringapparatus. During sputtering, an electric field forms in the sputteringapparatus between the target and an anode. This electric field ionizes agas, such as argon, within the sputtering apparatus so as to form aplasma. Typically, a plasma sheath, or dark space, separates a positivecolumn of the plasma from the sputter target. This sheath has a certainthickness. Introduction of an Al₂O₃ inclusion on the surface of thetarget can distort the electric field so as to alter the shapes of thepositive plasma column and the plasma sheath.

Over time, electrical charges can build up in the vicinity of an Al₂O₃inclusion. When the electrical charge imbalance becomes sufficientlystrong, a high current density cathodic arc forms. The high currentdensity cathodic arc heats a small section of the target surface, oftensufficiently to melt the target material in that section. The arcpressure causes droplets of liquid metal to eject from the sputteringtarget surface at high velocity and strike an intended substrate, suchas a silicon chip. The droplets, or macroparticles, solidify on thesubstrate creating large defects thereon. These macroparticles can rangein size from less than 1 μm to greater than 50 μm in diameter and canreduce significantly device yields, for example, in integrated circuitmanufacturing.

Dielectric inclusions and surface layers have long been known to causearcing in plasma discharges as well as in vacuum spark gaps. Morerecently, research on arcing in sputtering plasmas has shown thatinclusion and surface oxide induced arcing causes molten metalmacroparticle ejection from aluminum sputtering targets producingparticle defects on the substrate. High-speed video analysis of arcingfrom heavily doped aluminum-aluminum oxide sputtering targets has shownthat the molten metal macroparticles ejected therefrom can have speedsof over 500 m/sec and temperatures of 3000 K.

It has been reported that dielectric inclusions between 0.10 and 10 μmcause arc initiation in a hydrogen plasma with 0.1 mA/cm² dischargecurrent; hydrogen pressures between 2.7 and 13 Pa; and a cathode biasbetween 100 and 500 volts. Also, arcing from aluminum targets sputteredin 10¹⁴ ions/cm³ argon, hydrogen and nitrogen plasmas with 1-μm diameterAl₂O₃ inclusions on the cathode surface has been reported. Finally,evidence for a critical size effect for arc initiation from inclusionsin hydrogen tokomak plasmas has been reported but critical inclusionsizes were not measured.

Notably, attempts have been made to reproduce the above results whereinclusion sizes were measured. As a result, it was determined that thearcs were a result of surface contamination and not the size of theinclusions. Apparently, the small values for the critical size of theinclusion for arc initiation that initially were reported appeared to bedue to surface contamination effects. As such, it is important toprovide contaminant-free sputter targets when examining the effect ofinclusion sizes on arc initiation.

Consequently, there remains a need in the art for methods to inhibitbipolar arcing in Al or Al alloy sputter targets having Al₂O₃inclusions. Such methods are calculated to improve device yields anddecrease scrap, thereby reducing manufacturing costs in fields such asthe manufacture of integrated circuits.

SUMMARY OF THE INVENTION

The present invention provides a method for determining a critical sizefor an Al₂O₃ inclusion in an Al or Al alloy sputter target to preventarcing during sputtering thereof.

This method includes providing a sputtering apparatus having a plasmacolumn, such as argon. The plasma has a plasma sheath of a knownthickness during sputtering under a selected sputtering environment ofan Al or Al alloy sputter target having an Al₂O₃ inclusion-freesputtering surface. If the thickness of the sheath is unknown, itpreferably is measured by providing an Al or Al alloy sputter targethaving an Al₂O₃ inclusion-free sputtering surface for sputtering under aselected sputtering environment in the sputtering apparatus. The sheaththickness can then be calculated using the Child-Langmuir law byfactoring in the known conditions for the selected sputteringenvironment including the sputtering voltage, ion mass and ion currentdensity.

When the thickness of the sheath is known, or measured, for a selectedsputtering environment, the critical size of an Al₂O₃ inclusion can bedetermined based upon the thickness of the sheath. More specifically,the diameter, or effective diameter, of an Al₂O₃ inclusion in an Al orAl alloy sputter target must be less than the thickness of the plasmasheath during sputtering under the selected sputtering environment toinhibit arcing. Once the critical size is determined for the Al₂O₃inclusion, an Al or Al alloy sputter target having an Al₂O₃ inclusion ofa known diameter that is less than the thickness of the plasma sheathcan be provided for sputtering in the sputtering apparatus under theselected sputtering environment so that bipolar arcing of the Al or Alalloy sputter target is inhibited. Since sputtering of the sputtertarget is performed at the sputtering surface, it is preferred that themeasurement of the diameter of the Al₂O₃ inclusion be taken in a planesubstantially parallel with the sputtering surface.

Being able to determine the critical size of an Al₂O₃ inclusion in Al orAl alloy sputter target, will allow device yields, for example, inintegrated circuit manufacturing, to increase and will allow scrappedproducts to be reduced resulting in a monetary savings for all involved.

Accordingly, one object of the present invention is to determine thecritical size for an Al₂O₃ inclusion in an aluminum or aluminum alloysputter target sputtered under plasma in a sputtering apparatus.

Another object of the invention is to prevent arcing during sputteringof an Al or Al alloy sputter target having an Al₂O₃ inclusion.

Yet, another object of the invention is to understand how the inclusionor surface oxide size affects the propensity of the sputtering plasma toarc, as well as to find the relationship between inclusion size,sputtering power, and the propensity for arcing and molten macroparticleejection.

Other objects and advantages of the invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a conventional sputter target assemblyhaving a backing plate and a target;

FIG. 2 is a schematic perspective view of an aluminum target having asputter track provided with an aluminum oxide inclusion;

FIG. 3 is a partial top perspective view of an aluminum sputter targetprovided with an aluminum oxide inclusion;

FIG. 4 is a view of FIG. 3 after sputtering of the sputter target;

FIG. 5 is a partial top perspective view of an aluminum microparticle ona silicon wafer;

FIG. 6 is a graph representing the variation in total particle densityfound on silicon wafers with sputtering power density and inclusionsize;

FIG. 7 is a graph representing the particle size distributions found onsilicon wafers after sputtering at 8, 16, 24, and 32 W/cm² powerdensities with a 2940 μm inclusion in the target sputter track;

FIG. 8 is a graph representing the variation in total particle defectdensity on the wafer after sputtering with embedded inclusion size forsputtering power densities of 8, 16, 24 and 32 W/cm²;

FIG. 9 is a graph representing the arc rate as a function of embeddedAl₂O₃ inclusion size at 8, 16, 24 and 32 W/cm² sputtering powerdensities;

FIG. 10 is a graph representing the variation in inclusion critical sizewith sputtering power density for particle density and arc ratemeasurements;

FIG. 11 is a graph representing the correlation between arc rate andtotal particle defect density found on the wafer; and

FIG. 12 is a schematic diagram of the distortion of the plasma in asputtering apparatus by inclusion charging caused by an Al₂O₃ inclusionlocated on the surface of a sputter target.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

We have discovered unexpectedly that there is a critical size for anAl₂O₃ inclusion for initiating arcing and molten macroparticle emissionduring sputtering of Al or Al alloy sputter targets in a plasma in asputtering apparatus. As shown in FIG. 1, a sputter target assembly 10preferably includes a backing plate 12 and a target 14 bonded togetherwherein the target 14 and backing plate 12 both are made of Al or Alalloys. The target 14 includes a sputtering surface 16 for sputtering ina sputtering apparatus (not shown) and preferably comprises a high gradeAl or Al alloy. Preferred Al alloys for use in the target 14 and thebacking plate 12 include Al—Cu alloys. An intermediate layer (not shown)may be provided between the target and backing plate.

Any conventionally available sputtering apparatus used for sputteringtargets in a plasma, preferably an argon plasma, can be used. Theplasmas in these conventional apparatuses form a plasma sheath having aknown, or measurable, thickness under a selected sputtering environment.If the thickness of the sheath is unknown, it is preferably measured byproviding a contaminant-free Al or Al alloy sputter target having anAl₂O₃ inclusion-free sputtering surface for sputtering under a selectedsputtering environment in the sputtering apparatus. The sheath thicknesscan then be estimated using the Child-Langmuir law by factoring in theknown conditions for the selected sputtering environment including thesputtering voltage, ion mass, and ion current density among others. Morespecifically, sheath thickness, s in MKS units becomes s=4.7×10⁻¹¹V^(3/4)/(M_(i)·J_(i))^(1/4) with V representing the sputtering voltage;M_(i), the ion mass; and J_(i), the ion current density.

When the thickness of the sheath is known, or measured, for a selectedsputtering environment, the critical size of an Al₂O₃ inclusion can bedetermined based upon the thickness of the sheath. More specifically,the diameter, or minimum effective diameter, of an Al₂O₃ inclusion in anAl or Al alloy sputter target must be less than the thickness of theplasma sheath during sputtering under the selected sputteringenvironment to inhibit arcing. While some inclusions may besubstantially circular in nature such that a diameter measurement easilycan be realized, it is understood that a minimum effective diameter canbe realized for Al₂O₃ inclusions having somewhat irregular ornon-circular shapes. Accordingly, diameter shall be used interchangeablywith minimum effective diameter. It is this measured diameter that iscompared with the measured thickness of the plasma sheath. Additionally,since sputtering of the sputter target is performed at the sputteringsurface, it is preferred that the measurement of the diameter of anAl₂O₃ inclusion be taken along a plane substantially parallel with thesputtering surface.

Al₂O₃ inclusions that are located on the sputtering surface of an Al orAl alloy sputter target can be measured rather simplistically such asvia scanning electron microscopy, or any other suitable measuring deviceor means. In contrast, the inclusions provided within the sputtertarget, and not on the surface, must be measured by other more involvedmethods such as via an ultrasonic scanning method like the one disclosedin U.S. Pat. No. 5,406,850 hereby incorporated by reference herein.Additional measuring methods which may be suitable can be found in U.S.Provisional patent application Ser. No. 09/700,268 filed Nov. 9, 2000titled “Method and Apparatus for Quantitative Sputter Target Cleanlinessand Characterization, U.S. Provisional Patent Application Ser. No.60/197,790 filed Apr. 14, 2000 titled “Macroparticle Free MetallicSputtering Targets,” U.S. Provisional Patent Application Ser. No.60/311,152 filed Aug. 9, 2001 titled “Method and Apparatus forNon-Destructive Target Cleanliness and Characterization by Types ofFlaws Sorted by Size and Location” and International Application NumberPCT/US01/14403 filed May 4, 2001 titled “Cleanliness Evaluation inSputter Targets using Phase” all of which are hereby incorporated byreference herein.

When an Al₂O₃ inclusion has a diameter less than the sheath thickness,arcing behavior is inhibited when the inclusion is exposed to thesputtering plasma. When the inclusion has a diameter that issubstantially equal to or greater than the plasma sheath, then bipolararcing will occur causing significant arcing activity during sputteringwith ejection of molten metal macroparticles from the interaction of thecharged dielectric inclusion surface with the sputtering plasma. Assuch, if the Al₂O₃ inclusions in the Al or Al alloy sputter target aretoo large in diameter, the target is rejected.

Once the critical size is determined for the Al₂O₃ inclusion, acontaminate-free Al or Al alloy sputter target can be provided having noAl₂O₃ inclusions with diameters greater than the measured thickness ofthe plasma sheath for sputtering in the sputtering apparatus under theselected sputtering environment. Finally, the sputter target can besputtered in the sputtering apparatus under the selected sputteringenvironment, whereby arcing is inhibited.

EXAMPLE

The following example is provided for illustrative purposes only.

I. Experimental Procedures

To understand how the size of the Al₂O₃ inclusion in an Al or Al alloysputter target affects the propensity for the plasma in a sputteringapparatus to breakdown into an arc during sputtering, the followingsputtering experiments were conducted in a cryopumped vacuum chamberusing a 7.6 cm diameter aluminum sputtering target. The thickness of theplasma sheath in this study was controlled and determined to be between300 and 600 μm during sputtering of the aluminum target under a selectedsputtering environment as discussed below.

To begin, small holes were drilled into the face of sputtering targetsand small (0.01 to 3 mm) Al₂O₃ inclusions were placed into the holesusing tweezers. For purposes of the experiments herein, only Al₂O₃inclusions having an aspect ratio of about 1 were tested.

The softness of the aluminum and hardness of the Al₂O₃ inclusionsallowed the inclusions to remain mechanically locked in the sputteringsurfaces of the aluminum targets. The inclusion particles were alwaysplaced in the centers of the magneton erosion or sputter tracks on thetargets where sputtering occurred with more frequency.

FIG. 2 shows a schematic diagram of the experimental arrangement wherean aluminum target plate 18 has an embedded Al₂O₃ inclusion 20 in thesputtering surface 21. Each Al target plate 18 was placed into asputtering apparatus (not shown) and sputtered under a sputteringenvironment where the powers ranged from 8 W/cm² to 60 W/cm² at a 0.5 Paargon pressure. The argon plasmas so generated were magneticallycontained so that the targets operated in the DC magnetron-sputteringmode. The 10 kW switching sputtering power supply used in this study wasan Advanced Energy model 2012-061-m MDX–10K. Target/substrate spacingwas 143 mm. Sputtering power density was calculated by dividing thesputtering power by the area of the sputtering track 23, which was 12.7cm² for each target.

Table 1 below provides the sputtering voltage, current, ion currentdensity and the sheath thickness for the sputtering conditions used inthis study.

Power Density V I J_(i) s (W/cm²) (Volts) (Amps) (A/m²) (mm) 8 405 0.26194 0.60 16 459 0.46 344 0.49 24 486 0.64 479 0.44 32 496 0.82 613 0.3940 505 1.00 748 0.36 48 518 1.16 868 0.34 56 503 1.40 1047 0.30

The ion current density was calculated by assuming a secondary electroncoefficient of 0.045. The sheath thickness in the sputter track wascalculated using the Child-Langmuir Law. From Table 1, the sheaththickness for the experimental conditions used in this study decreasesfrom 600 μm at low power conditions to 300 μm at high power conditions.Sheath thickness for 24 W/cm² sputtering power was 440 μm.

FIGS. 3 and 4 show an enlarged ˜3 mm size Al₂O₃ inclusion 22 embeddedinto the sputtering surface 24 of an aluminum target plate 26 before andafter sputtering. As shown in FIG. 4, the inclusion 22 remains wellattached to the sputtering surface 24 with little change in size orshape after sputtering. The sputtering process erodes the sputteringsurface 24 so that some grain boundary structure 28 can be seen aftersputtering as suggested in FIG. 4.

Polished 150 mm diameter silicon wafers were used as the substrates forthese experiments. Deposited films were analyzed for particulate defectsusing a Tencor 6420 surface analyzer. Each wafer was analyzed aftersputter coating for 1 minute. Particulate defects in the deposited filmwere grouped by apparent size with size bins ranging from 0.5 to 12 μm.The background particle density was measured by depositing aluminumfilms on wafers without an embedded inclusion. The background totalparticle density was 8.5 cm⁻².

The arc rate was measured by monitoring detected current pulses on themain power lead from the power supply to the sputtering cathode. A coilplaced around the cathode power cable inductively detected the currentpulses when arcs occurred. The voltage pulses induced in the sensingcoil were recorded on an oscilloscope and a pulse counter. The arc ratewas the total arc count divided by the deposition time.

II. Results

Molten macroparticle ejection during arcing of sputtering targetsproduces defects that have a particular morphology arising from theirmolten origin. FIG. 5 shows aluminum macroparticles 30 captured on asilicon wafer 32 during aluminum sputter deposition. The relative largevolume of the large macroparticle 34 and the flattened shape shown inFIG. 5 are often seen in large macroparticles. Macroparticles 30 emittedduring the early stages of an arc or emitted as a result of surfacecontamination tend to be smaller and more spherical in shape. The shapesof the macroparticles 30 after solidification depend upon the surfacetension of the liquid and the diameter of the macroparticle dropletsprior to freezing on the wafer 32.

Measurements of the total macroparticle densities found on the wafersafter sputtering for 1 minute with various size inclusions embedded intothe faces of the target erosion tracks at various sputtering powers areprovided in Table 2 below and graphically in FIG. 6.

Inclusion Size (μm) Power 130 340 450 460 1200 2940 Density R_(a) P_(d)R_(a) P_(d) R_(a) P_(d) R_(a) P_(d) R_(a) P_(d) R_(a) P_(d) (W/cm²)(sec⁻¹) (cm⁻²) (sec⁻¹) (cm⁻²) (sec⁻¹) (cm⁻²) (sec⁻¹) (cm⁻²) (sec⁻¹)(cm⁻²) (sec⁻¹) (cm⁻²) 8 0 3.9 0 0 7.0 0 2.5 0 6.6 5 17 16 0 2.7 0 0.23.8 0.9 3.4 0.6 38 433 72 24 0 3.0 0 3.4 3.1 99 16 356 59 10718 368 321.6 0 1.4 81 19 914 39 2456 269 8934 377 40 0 2.2 48 0 3.5 56 0 12

In Table 2 above, R_(a) and P_(d) represent the arc rate and themeasured density of particles found on the wafer. At low sputteringpower density, no macroparticles were detected above the backgroundlevel. Inclusions with diameters of less than 340 μm did not causeejection of measurable macroparticles even at power densities of over 50W/cm². As the diameters of the inclusions increased from 340 μm, thetotal particle densities found on the wafers increased. Also, thesputtering power density required to generate particle defects on thewafers decreased to the point that for an inclusion having a diameter of2940 μm, a significant increase in particle defect density on the waferwas found with a sputtering power density of only 16 W/cm².

So from this data, it can be concluded that there are two factorscontrolling the emission of macroparticles from a sputtering target: thesputtering plasma conditions and the size of dielectric inclusions. Whenthe diameters of the dielectric inclusion falls below a critical value,arcing and macroparticle emission do not occur.

The size distributions of the macroparticle defects on the wafers aftersputtering with 2940 μm inclusions in the target surfaces is provided inFIG. 7. From this data, as the sputtering power density increased, thenumber of particle defects on the wafer increased in all sizecategories. Also, the majority of the particle defects (>60%) were lessthan 1.1 μm in size. This type of macroparticle size distribution isconsistent with measurements of macroparticle size distributions emittedby vacuum arc studies with clean metallic surfaces. During the arc, mostof the particles emitted are small, fast-moving particles emitted at anangle of 30° from the plane of the cathode, but a small number of verylarge slow moving particles are emitted with trajectories nearperpendicular to the cathode surface. These are the types of particlesshown in FIG. 5 and represented in FIG. 7 in the large size end of thesize distribution.

FIG. 8 shows how the total particle densities on the wafers varied withembedded inclusion size as the sputtering power density increased from 8to 32 W/cm2. Below about 500 μm inclusion size, the defect particledensity could not be distinguished above the background particle densityon the wafer. The critical inclusion size for generation ofmacroparticles on the wafer above the background level was determinedfor each power density by doing a least-squares fit of the data forinclusion sizes greater than 340 μm. The lines in FIG. 8 show theleast-squares fits of the data for the various power densities. It isclear that the slopes of the particle density versus inclusion sizelines increases with sputtering power. However, the x-intercepts do notchange significantly with sputtering power. All x-axis intercepts appearto be near the 500-μm-inclusion size.

A similar result is obtained if we plot the arc rate as a function ofinclusion size. This data is shown in FIG. 9. Again, as the powerdensity increases the slope of the arc rate versus inclusion size curvesincrease while the x-intercept remains relatively constant. In the caseof the curves at 24 and 32 W/cm², the slopes of both curves are equalwithin experimental error. This suggests that the arc rate may bereaching a saturation condition at 24 W/cm² so that increasingsputtering power density does not generate an increase in arc rate atthese high power densities.

The critical inclusion size for initiation of arcing and moltenmacroparticles ejection for each data set is graphed in FIG. 10 as afunction of the sputtering power density. The error bars are derivedfrom the error of estimate obtained from the least-squares-fits in FIGS.8 and 9. The data in FIG. 10 gives an average value for the criticalinclusion size of 440±160 μm. It can also be seen in FIG. 10 that thecrucial inclusion size is within experimental error independent of thesputtering power density. The critical size is 440±160 μm for theconditions used in this study wherein the plasma sheath thickness rangedfrom 300 to 600 μm.

These single inclusion experiments were repeated using a commercialsputtering source used for aluminum alloy deposition on 200-mm diametersilicon wafers. This sputtering source used a rotating magnet for plasmaconfinement and improved film uniformity and target utilization.Inclusions with sizes of 450 and 730 μm were embedded into the aluminumsputtering target surface and the target was sputtered at 10.6 kW powerwith 0.5 Pa argon pressure. Measurements of the arc rate and particledefect density on 200 mm silicon wafers after sputtering for 1 minuteshowed results consistent with the data reported herein for the 7.6 cmdiameter target. The arc rate was 2.5 sec⁻¹ with the 450-μm inclusionincreasing to 52 sec⁻¹ when the 730-μm inclusion was embedded.Similarly, the total particle defect density detected on the siliconwafer was 3 cm⁻² when the 450-μm inclusion was used increasing to 11cm⁻² for the 730 μm inclusion. These values fit the data in FIGS. 9 and10 for power density of 24 W/cm².

III. Discussion

In light of the above, the critical inclusion size appears to be relatedto the conditions for initiating the arc and not related to conditionsto form molten metal on the target surface or to break the molten metalsurface tension for ejection of molten macroparticles from the cathodesurface. It has already been shown that the force exerted by an arc onthe cathode surface is more than sufficient to overcome the surfacetension and eject molten macroparticles. So, once an arc occursmacroparticle ejection is likely. This is confirmed by comparing ourmeasurements of particle defect density on the wafer with arc rate. Thearc rate and the total particle defect density are clearly related toeach other. FIG. 11 shows the relationship between total particle defectdensity found on the wafer, P_(d), and the arc rate, R_(a), during filmdeposition. For the target-substrate geometry used in this study, at arcrates below about 2500 sec⁻¹ one particle is detected on the wafer forevery ten arc events.

Without wishing to be bound to any theory of operation, arc formationand ejection of the molten metal macroparticles appears to be based uponthe interaction of the dielectric surface of the inclusion with thesputtering plasma. FIG. 12 shows a schematic representation of theplasma 36 when an Al₂O₃ inclusion 38 is present on the cathodesputtering surface 40 of a target plate 42 such that the diameter (d) ofthe Al₂O₃ inclusion 38 is greater than the thickness (s) of the plasmasheath 44 that is located over an area of the target plate 42 free fromAl₂O₃ inclusions. Notably, the plasma sheath 44 separates the positivecolumn of the plasma 36 and the cathode sputtering surface 40.Introduction of the inclusion 38 on the sputtering surface 40 of thetarget plate 42 leads to distortion of the plasma 36 and the sheath 44.Since sputtering of the target plate 42 is performed at the sputteringsurface 40, it is preferred that the measurement of the diameter (d) ofthe Al₂O₃ inclusion 38 be taken along a plane 45 substantially parallelwith the sputtering surface 40.

Detailed examination of the process that leads to sheath disruption andthe formation of the arc, as represented by arrows 46 in FIG. 12, beginswith the charging of the dielectric surface layer from ion bombardmentduring the sputtering process. This charge distorts the electric fieldin the dark space about the dielectric region. The severity of thiselectric field distortion depends on the relationship between diameter(d) of the dielectric inclusion and the sheath thickness (s). For smalldiameter inclusions, when the inclusion exposed to the plasma issignificantly less than the sheath thickness, the disruption to theelectric field in the dark space is concentrated in the vicinity of theinclusion. In fact, when the dielectric inclusion size exposed to theplasma is much less than the plasma sheath thickness (s0, thedisturbance of the plasma sheath above and away from the inclusionsurface is negligible.

As the inclusion 38 exposed to the plasma 36 approaches a diameter (d)that is equal to or greater than the sheath thickness (s), the charge onthe inclusion 38 acts to almost neutralize the electric field in theoriginal sheath region above the inclusion 38. This causes the plasmaboundary to sag toward the inclusion 38 and the plasma sheath thicknessover the inclusion 38 decreases as the plasma positive column diffusesinto the volume over the charged inclusion 38. As the plasma columndiffuses inward and the field barrier separating the plasma 36 from thecathode shrinks, a plasma channel forms over the inclusion. This plasmachannel grows primarily by radial diffusion of the plasma 36.

Since one cannot expect the inclusion to be perfectly symmetrical, theelectric field distribution around the inclusion also will not besymmetrical. Since the electric field distortion will tend to mimic theinclusion asymmetry, the radial growth of the plasma channel will alsobe asymmetrical. As the plasma channel grows, a point is reached whereconditions permit breakdown and an arc strikes between a site on thecathode surface near the inclusion and the plasma channel. When the arcoccurs, the energy stored in the power supply (not shown) and theconnecting cable (not shown) is discharged as an arc. This arc, onceformed, is free to move across the cathode surface 40 creating arctracks and ejecting molten macroparticles.

As described above, the arc spot with this model will tend to occur onthe metallic cathode surface 40 near the inclusion 38. This isconsistent with observations that the arc tracks do not intersect theinclusions 38, but are near the inclusion 38.

If this model is correct, then we would expect that when the aspectratio of the inclusion 38 deviates from one, the smaller dimension ofthe inclusion 38 will control the field distribution distortion over theinclusion 38 and therefore will control the degree of sheath distortionand the propensity for arcing. So, from our model, the aspect ratio ofthe inclusion 38 will be an important variable. This model predicts thatthe arc rate will also be relatively insensitive to the area of theinclusion 38. For example, a very narrow inclusion with large area willarc at the same rate as an inclusion with an aspect ratio of 1 anddimension equal to the narrow dimension of the much larger area narrowinclusion.

Also, the arc rate will depend on location of the inclusion with respectto the local current density and sheath thickness. That is, if thesputtering surface 40 corresponds to an imaginary surface (not shown)within the sputtering apparatus, the arc rate and, thus, the localcritical diameter, will depend on location along that imaginary surface.Since for typical fixed magnet planar magnetron sputtering systems thecurrent density over the surface of the target is highly non-uniform,the sheath thickness will also be non-uniform depending upon the localcurrent density. From the Child-Langmuir law the sheath thickness in,for example, the sputter track area of a typical sputtering conditionwith 50 mA/cm² current density will be 0.4 mm. However, moving away fromthe sputter track to the areas where current densities decrease to 10mA/cm² will increase the sheath thickness to 0.9 mm. Therefore, thecritical inclusion size for arcing in fixed magnet systems depends onthe location of the inclusion 38 on the target surface 40. The criticalinclusion size decreases in the sputter track areas (high power densityand smaller sheath thickness) and increases in non-sputter track areas(low power density and larger sheath thickness). Lastly, when a thininclusion is present on the sputtering target surface, arcing can stillresult indicating that the thickness aspect of the inclusion does notappear to play a significant role in the process.

While the methods herein described constitute preferred embodiments ofthis invention, it is to be understood that the invention is not limitedto these precise methods and that changes may be made without departingfrom the scope of the invention, which is defined in the appendedclaims.

1. A method for selecting one or more Al or Al alloy sputtering targetsfor use in a sputtering apparatus comprising: a) determining a plasmasheath thickness in said sputtering apparatus; b) determining diametersof Al₂O₃ surface inclusions along a surface of each target of aplurality of Al or Al alloy sputtering targets; and c) selecting onlythose sputtering targets of the plurality of Al or Al alloy sputteringtargets having such diameters of Al₂O₃ surface inclusions less than saidplasma sheath thickness.
 2. The method as recited in claim 1 whereinsaid step a) includes sputtering an Al or Al alloy sputtering targetsubstantially free of Al₂O₃ inclusions in said sputtering apparatus in agas defining an ion mass at a sputtering voltage; determining said ionmass; determining said sputtering voltage; determining an ion currentdensity at a surface of said Al or Al alloy sputtering target; andestimating said plasma sheath thickness using said ion mass, saidsputtering voltage, and said ion current density.
 3. The method asrecited in claim 1 wherein said step a) includes sputtering an Al or Alalloy sputter target substantially free of Al₂O₃ inclusions in saidsputtering apparatus in a gas defining an ion mass M_(i) at a sputteringvoltage V; determining said ion mass M_(i); determining said sputteringvoltage V; determining an ion current density J_(i) at a surface of saidAl or Al alloy sputtering target; and estimating said plasma sheaththickness s using the equation s=4.7×10⁻¹¹ V^(3/4)/(M_(i)·J_(i))^(1/4).4. The method as recited in claim 1 wherein said plasma sheath thicknessis between about 300 μm and 600 μm.
 5. The method as recited in claim 1wherein said sputtering apparatus is capable of sputtering in an argonatmosphere at a sputtering power of between about 8 W/cm² and 60 W/cm².6. A method to inhibit arcing in an Al or Al alloy sputter target havingan Al₂O₃ inclusion comprising: a) providing a sputtering apparatushaving a plasma with a plasma sheath of a known thickness duringsputtering, under a selected sputtering environment, of an Al or Alalloy sputter target having an Al₂O₃ inclusion-free sputtering surface;b) providing an Al or Al alloy sputter target having one or more Al₂O₃inclusions and a sputtering surface for sputtering in said sputteringapparatus under said selected sputtering environment wherein said one ormore Al₂O₃ inclusions include a diameter less than said known thicknessof said plasma sheath; and c) sputtering in said sputtering apparatusunder said selected sputtering environment said Al or Al alloy sputtertarget having said one or more Al₂O₃ inclusions whereby arcing isinhibited.
 7. A method to inhibit arcing in an Al or Al alloy sputtertarget having an Al₂O₃ inclusion as recited in claim 6 wherein saidplasma is argon.
 8. A method to inhibit arcing in an Al or Al alloysputter target having an Al₂O₃ inclusion as recited in claim 6 whereinsaid diameter of said one or more Al₂O₃ inclusions is situated in aplane substantially parallel with said sputtering surface of said Al orAl alloy sputter target having said one or more Al₂O₃ inclusions.
 9. Amethod to inhibit arcing in an Al or Al alloy sputter target having anAl₂O₃ inclusion as recited in claim 6 wherein said one or more Al₂O₃inclusions have an aspect ratio of about
 1. 10. A method to inhibitarcing in an Al or Al alloy sputter target as recited in claim 6 whereinsaid Al or Al alloy sputter target having said Al₂O₃ inclusion-freesputtering surface and said Al or Al alloy sputter target with said oneor more Al₂O₃ inclusions are free from surface contamination.
 11. Amethod for determining a critical size for an Al₂O₃ inclusion in an Alor Al alloy sputter target as recited in claim 6 wherein said measuredthickness of said plasma sheath is between 300 μm and 600 μm.
 12. Amethod for determining a critical size for an Al₂O₃ inclusion in an Alor Al alloy sputter target as recited in claim 6 wherein said sputteringenvironment includes a 0.5 Pa argon pressure and a sputtering power from8 W/cm² to 60 W/cm².
 13. A method to inhibit arcing in an Al or Al alloysputter target having an Al₂O₃ inclusion as recited in step b) of claim6 wherein said Al or Al alloy sputter target is similar in shape andsize to said Al or Al alloy sputter target having said Al₂O₃inclusion-free sputtering surface.
 14. A method to inhibit arcing in anAl or Al alloy sputter target having an Al₂O₃ inclusion comprising: a)providing an Al or Al alloy sputter target having an Al₂O₃inclusion-free sputtering surface; b) providing a sputtering apparatusfor sputtering in a plasma said Al or Al alloy sputter having said Al₂O₃inclusion-free sputtering surface, said plasma including a plasma sheathhaving a certain thickness during sputtering of said Al₂O₃inclusion-free sputtering surface under a selected sputteringenvironment; c) measuring said thickness of said plasma sheath undersaid selected sputtering environment; d) providing one or more of an Alor Al alloy sputter target having one or more Al₂O₃ inclusions and asputtering surface for sputtering in said sputtering apparatus undersaid selected sputtering environment, said one or more Al₂O₃ inclusionsincluding a diameter; e) measuring said diameter of said one or moreAl₂O₃ inclusions; f) comparing said measured diameter with said measuredthickness of said plasma sheath; and g) sputtering in said sputteringapparatus under said selected sputtering environment at least one ofsaid one or more of said Al or Al alloy sputter target wherein saidmeasured diameter of said one or more Al₂O₃ inclusions is less than saidmeasured thickness of said plasma sheath so that arcing is inhibited.15. A method to inhibit arcing in an Al or Al alloy sputter targethaving an Al₂O₃ inclusion as recited in claim 14 further includingbetween steps f) and g) placing each of said one or more of said Al orAl alloy sputter target into a class of accepted and rejected sputtertargets, said class of accepted sputter targets including said each ofsaid one or more of said Al or Al alloy sputter target having said oneor more Al₂O₃ inclusions with said measured diameter being less thansaid measured thickness of said plasma sheath.
 16. A method to inhibitarcing in an Al or Al alloy sputter target having an Al₂O₃ inclusion asrecited in claim 15 further including after placing said each of saidone or more of said Al or Al alloy sputter target into said class ofaccepted and rejected sputter targets, rejecting each said Al or Alsputter target in said class of rejected sputter targets.
 17. A methodto inhibit arcing in an Al or Al alloy sputter target as recited inclaim 14 wherein said Al or Al alloy sputter target having said Al₂O₃inclusion-free sputtering surface and said one or more of said Al or Alalloy sputter target are free from surface contamination.
 18. A methodto inhibit arcing in an Al or Al alloy sputter target as recited inclaim 14 wherein said diameter of said one or more Al₂O₃ inclusions issituated in a plane substantially parallel with said sputtering surfaceof said Al or Al alloy sputter target.
 19. A method to inhibit arcing inan Al or Al alloy sputter target having an Al₂O₃ inclusion as recited inclaim 14 wherein measuring said thickness of said plasma sheath in saidstep c) includes applying a Child-Langmuir law.
 20. A method to inhibitarcing in an Al or Al alloy sputter target having an Al₂O₃ inclusion asrecited in claim 14 wherein said measured thickness of said plasmasheath is between 300 μm and 600 μm.
 21. A method to inhibit arcing inan Al or Al alloy sputter target having an Al₂O₃ inclusion as recited inclaim 14 wherein said sputtering environment includes a 0.5 Pa argonpressure and a sputtering power from 8 W/cm² to 60 W/cm².
 22. A methodto inhibit arcing in an Al or Al alloy sputter target having an Al₂O₃inclusion as recited in step d) of claim 14 wherein each of said one ormore of said Al or Al alloy sputter target is similar in shape and sizeto said Al or Al alloy sputter target having said Al₂O₃ inclusion-freesputtering surface.